complete CustomerGroups implementation

[michaelbromley]

Is your feature request related to a problem? Please describe.
In our data model and Admin API we have CustomerGroups, but there is no UI to administer them, there are no e2e tests and little integration.

Describe the solution you'd like

1. Create Admin UI components to administer customer groups. This would likely work well if presented in the same way as we do with the ZoneList, since the CustomerGroup entity only has a single own property - "name".
2. Write e2e tests covering the basic CRUD scenarios for customer groups.
3. Create a PromotionCondition based on the customer belonging to given groups
4. Create a ShippingEligibilityChecker based on the customer belonging to given groups